The eyelid, commonly known as the eyelid, is located in front of the eye and forms the barrier that protects the eye. The eyelid is divided into an upper lid and a lower lid, and the fissure between the upper and lower lids is called the lid fissure. The fissure between the upper and lower lids is called the lid fissure. The upper lid is classified according to the number of folds and creases: 1) Single lid: the upper lid skin is smooth with no folds, commonly called monolid; 2) Heavy lid: the upper lid skin has a skin fold near the upper lid margin, commonly called double eyelid; 3) Multiple lids: the upper lid skin has multiple folds.